Microscopy methods, such as scanning tunnelling microscopy, require the use of probes having extremely sharp tips with well-defined shapes in order to provide a desired level of resolution for high quality images. A sharper probe, that is a probe with a narrower tip, provides higher resolution information about a sample while a well-defined probe shape lowers noise levels on resulting images.
Probes with sharp tips are known to be made using a process known as the “drop-off method”. In this process, an object to be etched, such as a piece of tungsten wire, has a lower portion immersed in an electrolyte such as sodium hydroxide or potassium hydroxide, while an upper portion of the piece of wire remains in air. The depth of immersion of the lower portion is chosen depending on a desired drop-off time, which governs the ultimate shape of the tips formed by the process. A ring-shaped electrode is placed around the immersed portion of the piece of wire and a voltage is applied between the piece of wire and the electrode.
An electrochemical reaction takes place between the piece of wire and hydroxide ions in the electrolyte, creating water molecules and molecules of, in the case of the object being made of tungsten, an oxidised compound of tungsten and oxygen called tungstate. As the portion of the tungsten piece surrounded by the electrode decomposes in this way, a neck of decreasing radius is formed. The rate of decomposition of this portion is inhomogeneous due to two effects: the formation of a meniscus of electrolyte around the piece at the surface of the electrolyte, and the accumulation of tungstate as it descends as a viscous flow near the immersed surface of the piece. The process culminates in the lower part of the piece falling away as the neck breaks, resulting in the formation of two sharp tips, the shapes of which are dependent on the rate of etching. A high rate of etching results in irregularly-shaped tips, and a low rate of etching results in very long and fragile tips. The applied voltage must be immediately terminated upon breakaway of the lower part to prevent further undesired etching taking place. Only one pair of tips can be made at a time in this manner, and in practice it is often the case that only one of the tips of the pair is usable.
The shape of the tips can be further affected by the behaviour of the meniscus. As the neck radius decreases and the surface area of the neck increases during the reaction, the meniscus position can change, which leads to the formation of a second neck. This causes undesired variations in the shapes of the final tips, rendering them unsuitable for use in very sensitive applications. Control of the apparatus is required to prevent this from happening.
Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages associated with the prior art.